The present invention relates to a document size detection apparatus for an image recording apparatus wherein a document size can be detected with a document holder having thereon no device.
There has been known an image recording apparatus wherein a document is placed on a document table, the document is held down by a document holder (hereinafter referred to as "a platen cover"), the document is irradiated by a lamp from the under side of the document table, the reflected light from the document is projected directly on a photoreceptor drum through a mirror and an optical system, or the reflected light is converted once to digital signals by means of a photoelectric converting element such as a CCD for giving necessary image processing, thus an electrostatic latent image is formed on the photoreceptor drum and the electrostatic latent image is developed for recording the document.
In some of the aforementioned image recording apparatuses, a size of a document to be recorded can be detected and a recording sheet whose size corresponds to the detected size is fed automatically.
Heretofore, there have been proposed various methods for detecting document sizes. For example, there has been known a method wherein plural optical sensors are provided at the locations under the document table corresponding to document sizes, and thereby the document size can be detected by output from the optical sensor. Further, in another example, the reverse side of the platen cover is blackened or mirror-finished so that the reflected light from a document may be different from that from non-document area when the document is held by the platen cover, and thereby the document size is detected by variation in the amount of light accepted by an optical sensor when the document is scanned (for example, Japanese Patent Examined Publication No. 64-2263 and Japanese Patent Publication Open to Public Inspection (Kokai) No. 54-83438 (hereinafter referred to as Japanese Patent O. P. I. Publication)).
In the detection method mentioned above, however, the amount of reflected light from the document is reduced in the case of a transparent document used for an OHP (overhead projector) or a thin document, being affected by the black or mirror-finished- reverse side of the platen cover. Therefore, the document can not be recorded clearly, or the record is hard to read even if it can be made. Further, in the detection method mentioned above, black streaks appear on the upper and lower margins or margins at the left side and the right side of the recording paper when the document is not placed accurately at the predetermined position on the document table. Therefore, it is necessary to eliminate the black streaks, and for this purpose, it is necessary to adjust accurately the sheet feed timing for the recording sheet and to prepare the signal processing circuit that prevents the occurrence of black streaks. Furthermore, when the recording sheet used is larger than the document black or mirror-finished areas around the document are recorded on the recording sheet, which is a problem.
Further, there has been known another detection method wherein yellow stripes are drawn on the reverse side of the platen cover and the document size is detected by the difference of color between the yellow stripe and white document. This method still has the same problems as in the foregoing when a transparent or thin document is used, and in the case of a full color image, the yellow stripes are regarded as information for recording and thereby are recorded, which is also a problem.
In any conventional document size detection methods, the reverse side of a platen cover has thereon a device for detecting document size, and in some cases, this device is regarded as document information and is recorded accidentally on a recording sheet. Further, in the case of an image recording apparatus employing an automatic document feeder (called an ADF) wherein plural documents are fed automatically to the predetermined exposure position one sheet at a time by means of a conveyance belt, it is not possible to provide a device for document size detection on the conveyance belt because the position of the conveyance belt changes each time of conveyance, which is a problem.
The present invention has been devised in view of the aforementioned points, and its first object is to detect the size of a document, with no device on a platen cover, by detecting shadows cast around the document on the platen cover when the document is irradiated.
However, when a position of the shadow is detected by judging whether the variation ratio of image signals obtained through reading document information exceeds the predetermined threshold value there is a fear that the position of the shadow is detected incorrectly because the variation ratio of image signals will increase in the area surrounding the document when the platen cover is soiled.
The present invention has been devised in view of the aforementioned points and its second object is to prevent the wrong detection of the document size caused by the soiled platen cover.
Further, when a position of the shadow of the document is detected by judging whether the variation ratio of image signals obtained through reading document information exceeds the predetermined threshold value, many variation points where the variation ratio of image signals exceeds the predetermined threshold value exist not only in the side edge area of the document but also in the area inside the document including document information, and such variation points further exist even in the area surrounding the document when the platen cover is soiled. Therefore, in the method wherein a position of the shadow is detected by detecting the variation point which first exists after investigating from the starting position for scanning and the ending position for scanning, there is a fear that the document size can not be detected accurately because of possible impossibility of making a reliable judgment.
The invention has been devised in view of the aforementioned points, and its third object is to detect the document size more accurately.